Question: 3 binders cost $3.87. Which equation would help determine the cost of 13 binders?
There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 3 binders. We want to know the cost of 13 binders. We can write the numbers of binders as a proportion: $\dfrac{3}{13}$ We know 3 binders costs $3.87. We can let $x$ represent the unknown cost of 13 binders. The proportion of these costs can be expressed as: $\dfrac{\$3.87}{x}$ The cost changes along with the number of binders purchased, and so the two proportions are equivalent. $\dfrac{3}{13} = \dfrac{\$3.87}{x}$